This invention relates to low profile surface mounted magnetic devices, such as inductors and transformers. In particular, it relates to such devices having controllable inductance versus current characteristics.
Magnetic devices, such as inductors and transformers, serve a wide variety of essential functions in many electronic devices. In power supplies, for example, inductors are used as choke coils for energy storage and to minimize noise and AC ripple, and transformers are used to change voltage level and to provide isolation. Such devices are often made of a magnetic core, such as iron or ferrite, wound with conductive coils. Consequently, they are sometimes referred to as wire-wound core devices.
One major difficulty with wire-wound core devices is that they have been difficult to miniaturize. While components such as resistors, diodes, capacitors and transistors have been shrunk to the microscopic level, wire-wound core devices remain bulky and typically must be assembled as complete units before being applied to hybrid circuits.
Conveniently fabricated magnetic devices with a low surface profile are described in U.S. Pat. No. 5,574,420 issued to A. Roy et al. on Nov. 12, 1996 and entitled xe2x80x9cLow Profile Surface Mounted Magnetic Devices and Components Thereforxe2x80x9d, which is incorporated herein by reference. In essence the devices comprise a crenelated magnetic ferrite body and a conductive path winding through recesses around the body and securing it to a substrate. The conductive path can be comprised of U-shaped conductive elements, each partially surrounding the body, and conductive strips printed on the substrate. An alternative low profile device wherein the conductive path is printed around the body is disclosed in U.S. Pat. No. 6,094, 123 issued to A. Roy on Jul. 25, 2000, which is also incorporated herein by reference.
Such devices can exhibit nonlinear inductance versus current characteristics. As the current through the winding path increases to some threshold current level, the device inductance decreases from a high inductance to a region of relatively constant inductance. This characteristic and the threshold current level are essentially fixed for each device. But for some applications it is desirable to have a device where the threshold current level can be controlled.
In accordance with the invention, a low profile magnetic device comprises a crelenated ferrite body having first and second conductive paths winding around the body. Current applied to one of the conductive windings can adjust the threshold current level in the other conductive winding at which the inductance characteristic changes to a relatively constant inductance.